To convey fluids which are subject to undesirable cooling (or heating), several types of insulated conduits have previously been devised. For protection of single pipes, conduits have been fabricated utilizing the pipe-within-a-pipe concept where a carrier pipe is located within a casing pipe and where the annular space between them is filled with insulation, such as glass fiber or rigid foam. Although some attempts have been made to install more than one pipe within a single exterior casing pipe, the more common approach to multiple pipe conduits is to fabricate a conduit in which the casing consists of a long box and the pipes are laid within it and are not directly insulated. To provide thermal protection to the pipes, these conduit boxes are filled with various types of loose fiber or granulated insulation. In instances where periodic access to the pipes is required, the walls of the conduit box are adequately insulated instead of using loose insulation fill, thus allowing the pipes to remain bare, and a removable side or top of the conduit box provides access to the pipes.
The object of all thermally insulated conduits is to minimize heat transfer by provision of an adequate thickness of insulation while keeping the exterior surface area of the conduit at a minimum, since both the thicker insulation and the smaller surface area reduce the rate of heat transfer. In this respect, for a single carrier pipe, the pipe-within-a-pipe design provides the most efficient thermal design. However, when two or more fluid carrying pipes are installed within one casing pipe it becomes practically difficult or impossible to gain access to any of the individual fluid carrying pipes. Although thermally fairly efficient, this design, containing several pipes, is usually impractical. The alternate approach, involving insulation of individual pipes separately, requires more insulation and increases the external surface area considerably, making it thermally inefficient and less economical. The insulated box-type designs, which are the most common, achieve various degrees of efficiency depending on the cross-section of the box and the amount and type of insulation used, but they all are thermally less efficient than the pipe-within-a-pipe design. However, these box-type designs are more practical to install and to operate.
All these insulated conduits contain four basic functional components: fluid carrying pipes, thermal insulation, structural members, and protective cladding. In the box-type conduits all these components are separate and distinct, while in the pipe-within-a-pipe design the exterior pipe provides structural stability as well as acting as a protective cladding. In all designs economies are achieved by aiming at the most compact configuration of the conduit using small volumes of efficient insulation and reducing the size and number of structural members. Cladding is usually reduced in proportion to the other components and reduction of pipe diameter is only possible where the pipe material offers better flow characteristics.
Insulated conduits such as those used for municipal utility systems in the Arctic are installed above ground on supporting foundations such as piles. Spacing of the supports depends on the rigidity and strength of the conduit. Conduits which are structurally efficient allow longer spans to be used, thereby reducing the cost of supporting foundations. Therefore one of the criteria used in conduit design is the beam strength of the structure. In practise this beam strength is incorporated into the framework of the box, or it is provided by installation of longitudinal beams located beneath the box.
Most conduits of the types described have lent themselves to a very limited amount of component prefabrication which results in considerable labour requirements during on-site erection. Consequently the need for increased use of pre-fabrication has been felt for some time, particularly in remote regions where labour costs are high.